STORM-WAVE MOVEMENT OF MEGAGRAVEL IN WESTERN IRELAND: IMPLICATIONS FOR UNDERSTANDING BOTH STORMS AND TSUNAMI

Coastal boulder deposits (CBD) accumulate above the high water mark along high-energy seaboards at locations worldwide. CBD include isolated clasts on coastal platforms as well as sorted, imbricated boulder ridges at the inland extent of marine influence. The extent to which these deposits represent the effects of tsunami versus storm wave activity has been extensively debated in the literature. Ireland’s western coasts — where tsunami have not occurred in historical times — are the ideal place to study storm-dominated CBD.

Severe winter storms in 2013-2014 provided an opportunity to quantify the effects of high-energy marine bombardment on CBD in western Ireland. The pre-storm baseline was an extensive archive of precisely-located field photographs taken in the previous few years. Repeat photography and field measurement in summer 2014 documented movement of 1152 clasts on supratidal shore platforms and in boulder ridges. Of these, 13 had masses >50 t , and 67 were in the range 20-50 t. The largest block moved was >400 t.

The large-block movements inform the ongoing discussion of how to distinguish storm deposits from those of tsunami. Not only did these storm waves move a block with mass ≈400 t, they also transported several clasts (masses 36-49 t) horizontal distances of 75-100 m. These measurements mean that without other contextual evidence, the horizontal transport of megagravel cannot uniquely fingerprint paleotsunami events and cannot be used to inform tsunami chronologies. Transported clasts (as well as newly excavated boulders with masses up to 73 t) were added to imbricated boulder ridges, demonstrating that storm waves do contribute to imbricated CBD, and that boulder imbrication is also not diagnostic of tsunami deposition.

It’s important to note that although these were significant storms (central pressures in the range 962-940 mb, i.e. equivalent to hurricane Categories 3 and 4), they were not extreme meteorological events: bigger storms with bigger waves are likely to transport larger blocks and move them farther. Work is therefore needed to more finely calibrate the process sedimentology of storm-activated CBD, both to distinguish them from tsunami deposits and to understand high-energy coastal dynamics.